The IMT-Advanced (International Mobile Telecommunication Advanced) system is the fourth-generation mobile communication system evolved from the IMT-2000 system, and aims to provide improved mobile communication services.
In the IMT-Advanced system, it is presumed that a higher frequency band is allocated than in the IMT-2000 system. Accordingly, propagation loss in the IMT-Advanced system is significant compared to the IMT-2000 system, and, consequently, the cell coverage provided by one radio communication base station apparatus (“evolved Node B” or “eNB”) may be limited. Therefore, at present, in US IEEE 802.16j, European WINNER Project, and so on, studies are underway to prove a radio communication relay apparatus to expand eNB cell coverage and improve performance of a radio communication terminal apparatus (“user equipment” or “UE”) in a cell edge. Here, the radio communication relay apparatus studied in IEEE 802.16j is refereed to as “RS (Relay Station).”
In an eNB cell edge, radio waves from the nearest eNB are weak and it is not possible to expect a plurality of propagation paths (or paths) to be found, and therefore it is estimated that the SINR (Signal-to-Interference and Noise power Ratio) is also low. If the SINR is low, a modulation scheme with low transmission speed such as QPSK (Quadrature Phase Shift Keying) is adopted. However, by applying an RS to this radio wave environment, it is possible to provide radio waves of a strong electrical field to a UE that is located in an eNB cell edge, and improve the SINR. If the SINR is high, it is possible to adopt a high-order modulation scheme such as 16 QAM (16 Quadrature Amplitude Modulation) and 64 QAM (64 Quadrature Amplitude Modulation), and improve bit error performance. By this means, it is possible to reduce the number of times of error correction coding and perform communication using radio resources efficiently.
Also, in a general mobile communication system, ARQ (Automatic Repeat Request) is adopted where, when the receiving side receives packets with error, the transmitting side is requested to retransmit the packets. This ARQ is the technique of efficiently compensating packet error with less delay by performing retransmission processing of packet error, which occurs in a transmission period of communication lines, in the MAC layer of layer 2. In ARQ, the transmitting side attaches CRC (Cyclic Redundancy Check) bits for error detection to transmission data and transmits the result. The receiving side performs error detection by CRC bits attached to received data. Here, if packet error is not present in the received data, the receiving side feeds back the sequence number of that packet and an ACK (ACKnowledgement) signal to the transmitting side. By contrast, if packet error is present in the received data, the receiving side feeds back the sequence number of the packet in which error is detected, and a NACK (Negative ACKnowledgement) signal, and the transmitting side retransmits only that packet in which error is detected, to the receiving side. By this means, error that occurs in the transmission period is compensated.
Also, studies are underway to apply HARQ (Hybrid ARQ) combining ARQ and FEC (Forward Error Correction), to the IMT-Advanced system. Here, HARQ includes the chase combining method and IR (Incremental Redundancy) method.
The chase combining method is the technique of retransmitting the same packets upon retransmission as in the initial transmission and performing maximum ratio combining of the initial transmission packets and the retransmission packets to improve the SINR of received packets and reduce packet error efficiently. To be more specific, the transmitting side performs FEC coding and transmission of transmission data, and the receiving side performs FEC decoding processing of received data. Upon detecting packet error in the received data, the receiving side feeds back a NACK signal including the sequence number of the packets to the transmitting side and stores the packets in which error is detected, in a buffer (memory) of the receiving side. Next, the transmitting side retransmits the sequence number included in the feedback NACK signal to the receiving side, and the receiving side performs maximum ratio combining of the retransmitted packets and the packets (with error) stored in the buffer, and performs error correction again by FEC decoding processing.
Also, the IR method does not transmit all encoded data subjected to FEC coding upon the initial transmission, and sequentially transmits encoded data using puncturing processing (that punctures bits of encoded data to a data length in which the original bit sequence is reconfigurable in FEC decoding processing only by transmission). To be more specific, the IR method is the technique of efficiently compensating error by sequentially transmitting, on the transmitting side, encoded data punctured by puncturing processing (i.e. encoded data which is not transmitted) if the receiving side detects bit error.
Here, FIG. 1 shows an example of HARQ operations in the IMT-Advanced system.
First, upon directly transmitting data to the UE, the eNB transmits scheduling information indicating resource allocation information of DL (DownLink) data, to the UE and RS (step (“ST”) 1). The eNB transmits data (of sequence number #1) to the UE and RS, using a radio resource block indicated by the scheduling information (ST 2).
The UE and RS perform FEC decoding processing of the data transmitted from the eNB (ST 3-1 and ST 3-2). Here, the UE receives data with packet error and therefore transmits a NACK signal including sequence number #1 of the data to the eNB (ST 4-1). Also, the RS receives data without error, and therefore transmits an ACK signal to the eNB and stores data #1 in a buffer (memory) (ST 4-2).
Upon receiving a NACK signal from the UE, the eNB checks whether or not to have received an ACK signal from the RS. Further, if the eNB has received an ACK signal from the RS, the eNB requests the RS to retransmit data of sequence number #1 (i.e. retransmission request (#1)) to the UE (ST 5).
The RS requested to retransmit the data of sequence number #1 in ST 5 transmits data #1 stored in the buffer in ST 3-2 to the UE (ST 6). The UE performs a packet combination of data #1 (with packet error) received from the eNB in ST 2 and data #1 received from the RS in ST 6 (ST 7), and then performs FEC decoding processing of the combined packets again.
By this means, the UE can obtain a diversity effect by receiving the same data from a plurality of apparatuses (i.e. the eNB and RS) in a mobile communication system. Thus, by using HARQ, it is expected to provide reliable communication.
In IEEE 802.16j, the above HARQ method is referred to as “RS-Assisted-HARQ.”
Also, the IMT-Advanced system is predicted to support MBMS (Multimedia Broadcast/Multicast Service). Here, MBMS is a service developed for not only broadcasting multimedia data (hereinafter “MBMS data”) but also for multicasting MBMS data to UE's subscribing to a corresponding service. For example, MBMS provides news channel, music channel, movie channel, and so on.    Non-Patent Document 1: IEEE P802.16j/D1 “Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Multihop Relay Specification”, 2007-08